Nickel-chromium alloy for reformer tubes

ABSTRACT

CARBIDE-STRENGTHENED ALLOY CONTAINING NICKEL, CHROMIUM, MOLYBDENUM AND/OR TUNGSTEN, COBALT AND NIOBIUM PROVIDES COMBINATION OF HIGH STRESS-RUPTURE STRENGTH AT ELEVATED TEMPERATURES, RESISTANCE TO CORROSION BY COMBUSTION PRODUCTS OF HYDROCARBON FUELS, GOOD WELDABILITY AND GOOD CASTABILITY AND IS PARTICULARLY SATISFACTORY FOR USE IN PRODUCTION OF GAS PLANT APPARATUS, SUCH AS REFORMER TUBES.

Aug. 1, 1972 s. w. K. SHAW E AL 3,681,059

NICKEL-CHROMIUM ALLOY FOR REFORMER TUBES Filed Dec. 12, 1969 5 Sheets-Sheet 1 l 1 f 1} 00 l\ O V- W N N N Q awe/yer Aug. 1, 1972 5 w, sH w -ET AL 3,681,059

NICKEL-CHROMIUM ALLOY FOR REFORMER TUBES Filed Dec. 12, 1960 5 Sheets-Sheet 2 k Q s G to 0 v N T I v N 2 Q Q Q n N N l l l l N s; a 8 Q mwzwn ms Arm/9045') Aug. 1, 1972 S. W. K. SHAW ET AL NI()KIjIrCHROMIUM ALLOY FOR REPORMER TUBES Filed Dec. l 2, 19 69 lOOO 25% C;- ZZ/ b 5 Sheets-Sheet 3 \Szwler-Alurse 91? Aug. 1, 1972 s. w. K. SHAW ET AL NICKEL-CHROMIUM ALLOY FOR REFORMER TUBES 5 Sheets-Sheet 4 Filed D00. 12, 1969 30% C7; Ms

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w m m w m 38 O a c 0 a w M 1N0 0/9 V Aw/ m .w/ a

1, 1972 s. w. K. SHAW ETAL 3,681,059

NICKEL-CHROMIUM ALLOY FOR REFORMER TUBES 5 Sheets-Sheet 5 Filed Dec. 12, 1969 kwnw/ (woods-J0 QanJOWa -SsWaJ 000/ j flwm j 91/7 Jan/QM pspaug United States Patent @fice 3,681,059 Patented Aug. 1, 1972 3,681,059 NICKEL-CHROMIUM ALLOY FOR REFORMER TUBES Stuart Walter Ker Shaw, Coldfield, and Peter John Penrice, Birmingham, England, assignors to The International Nickel Company, Inc.

Filed Dec. 12, 1969, Ser. No. 884,670 Claims priority, application (grfiat Britain, Dec. 13, 1968,

US. Cl. 75-134 F 5 Claims ABSTRACT OF THE DISCLOSURE Carbide-strengthened alloy containing nickel, chromium, molybdenum and/or tungsten, cobalt and niobium provides combination of high stress-rupture strength at elevated temperatures, resistance to corrosion by combustion products of hydrocarbon fuels, good weldability and good castability and is particularly satisfactory for use in production of gas plant apparatus, such as reformer tubes.

The present invention relates to nickel-chromium alloys for use at high temperatures and more particularly to carbide-strengthened nickel-chromium alloys.

Many industrial processes, for instance, the reforming of hydrocarbons, are now carried out at temperatures of up to 1000 C. and above require plant apparatus and articles made of alloys that will withstand stress and corrosive attack at these temperatures. Alloys for such use should have good casting properties, especially for casting in air, so that components of the plant can be economically produced by casting, and should also be weldable.

In the reforming process hydrocarbons react with steam in the presence of a catalyst to form a mixture of hydrogen and carbon monoxide and this gas mixture is widely used, under the name synthesis gas, for the production of alcohol. It is also used as a source of hydrogen for ammonia production. The reforming reaction is carried out at high temperatures, e.g., from 800 C. to 1000 C., under elevated pressure in tubular reactors known as reformer tubes, which are heated externally by the combustion of hydrocarbon fuels.

Alloys for reformer tubes must have good resistance to creep and rupture under stress at the operating temperatures and must maintain adequate toughness when heated for prolonged periods. Reformer tube alloys must resist carburization and also corrosion by the combustion products of impure fuels, which can cause sulphidation attack on some alloys. Hitherto, reformer tubes have usually been made, by centrifugal casting, from 25% Cr-20% Ni steels (known by the Alloy Casting Institute designation HK), though higher nickel alloys are being considered for this use.

Although many attempts have been made to overcome the foregoing problems and others, there is still a need for alloys having an improved combination of high stressrupture strength along with satisfactory corrosion resistance, weldability and castability for production of reformer tubes.

There has now been discovered an air-castable nickelchromium alloy containing a specially controlled combination of carbon, carbide-forming elements and matrix elements which provides highly desirable strength, including stress-rupture strength and impact strength, along with good corrosion resistance for service in apparatus which must withstand long-time usage in corrosive environments at elevated temperatures, including gas reforming plant apparatus and other apparatus, e.g., glass-forming apparatus.

It is an object of the present invention to provide a nickel-chromium alloy having high strength and good corrosion resistance, particularly including resistance to detrimental carburization.

Another object of the invention is to provide high strength corrosion-resistant articles and structures, including castings, welded structures, reformer tubes and also spinners or spinnerettes for production of glass fibers.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawing in which:

FIG. 1 is an alloy chart having total percentages of percent molybdenum plus one-half times percent tungsten along the ordinate axis and percentages of chromium along the abscissa axis and illustrating specially correlated molybdenum and/or tungsten-chromium areas ABCDEFA and GHIJKLG.

FIG. 2 is an alloy chart having percentages of cobalt along the ordinate axis and percentages of chromium along the abscissa axis and illustrating, inter alia, specially correlated cobalt-chromium areas MUVPQM, MNOPQM and RSOTUR.

FIGS. 3 through 8 are charts depicting stress-rupture life, in hours, in relation to alloy composition proportions, e.g., percentages of cobalt, or niobium or total percentage of percent molybdenum plus one-half the percent tungsten.

Generally speaking the present invention contemplates an alloy containing 0.4% to 1.4% carbon, 0.5% to 5% niobium, 23.5% to 33% chromium, one or both of molybdenum and tungsten in amounts so correlated with the chromium content that the sum (percent M0)| /2 (percent W) is 1% to 9% and the composition of the alloy is represented by a point within the area ABCDEFA in FIG. 1 of the accompanying drawing, cobalt in an amount from 5% to 42% so correlated with the chromium that the composition of the alloy is represented by a point within the area MUVPQM in FIG. 2 of the accompanying drawing, up to 12% iron, up to 1% in total of titanium or aluminium or mixtures thereof, up to 1% zirconium and up to 0.1% boron. The balance of the composition, apart from impurities and incidental elements, is nickel. Alloy composition percentages referred to herein are by weight percent.

The high strength of the alloy at elevated temperatures is largely due to the presence of insoluble carbides. Carbon is therefore an essential constituent of the alloy, and for adequate stress-rupture strength at least 0.4% carbon must be present. As the carbon content is increased the stress-rupture strength at first increases but then decreases. Excessive amounts of carbon must also be avoided because as the carbon content increases the impact strength of the alloys decreases and advantageously the carbon content does not exceed 1%. The optimum carbon content depends on the manner in which the castings are made.

In industrial practice, reformer tubes are made by centrifugal casting in a metal mould preheated to 200 C. to 400 C. These casting conditions result in undirectional solidification radially inwards from the mould face, which promotes columnar crystallization, and the formation of a uniform dispersion of fine carbide particles. For castings made in this way, and also for those made by static chill-casting, the best combination of stress-rupture and impact strengths is obtained with carbon contents of 0.43% to 0.7%. In contrast to this the carbide particles in castings made in sand or other refractory moulds, where cooling takes place more slowly, are much coarser. Thus a higher carbon content is generally required to achieve a given stress-rupture life in such a casting than in a chillcast casting and optimum properties are obtained with sistent with the strength required, and is desirably less.

than 0.65%.

Niobium also contributes to the stress-rupture strength as a carbide-former, and at least 0.5% and advantageously at least 1% is present for this purpose. As the niobium content increases the stress-rupture strength passes through a maximum and then begins to fall. Hence the niobiumcontent must not exceed and more usefully it is not more than 4%. Most advantageously the niobium content is 1.5% to 3.5% to obtain high stress-rupture strength.

Tantalum may be introduced incidentally with the niobium in an amount up to one-tenth of the niobium content, but larger amounts of tantalum very adversely affect the stress-rupture life.

Chromium in the alloys confers resistance to corrosion, but to avoid the formation of an undesirable alpha-chromium phase the chromium content must not exceed 33%.

Tungsten and molybdenum contribute to the stress rupture strength of the alloy. The combined molybdenumtungsten content, expressed as percent M0+' /2 (percent W), must be at least 1%. As this content is increased at the expense of nickel in an alloy of otherwise fixed composition, the stress-rupture strength increases to a maximum and then falls. The value of percent Mo+% (percent W) at which this maximum is reached is inversely related to the chromium content, as indicated by the shape of the area ABCDEFA in FIG. 1. Advantageously, the value of percent Mo+ /2 (percent W) is at least 2%, and is so correlated with the chromium content that the composition of the alloy corresponds to a point within the area GHIJKLG.

At a given combined content of molybdenum and tungsten, the stress-rupture life also depends upon the relative proportions of molybdenum and tungsten, and beneficial ly the alloys contain at least 2% of tungsten.

Cobalt also contributes to the stress-rupture life, which rapidly increases to a maximum as the cobalt content increases in an alloy having the optimum combined molybdenum and tungsten content for given contents of chromium and niobium, and then decreases as the cobalt is further increased. The cobalt content at which the best properties are obtained increases with the chromium content, as indicated by the shape of the area MUVPQM in FIG. 2, and advantageously the composition of the alloys corresponds to a point in the area MNOPQM and most advantageously to a point in the area RSQTUR.

The alloys may contain small amounts of titanium or aluminum or both, up to a maximum of 1% in total. Titanium in these amounts has an advantageous effect on the tensile ductility provided the alloy is cast in such a way, e.g., centrifugally cast, that oxide inclusions are avoided, and aluminum does not impair the properties. Thus, centrifugally-cast tubes can contain up to 1% of titanium or aluminum, but statically-cast castings, which are more prone to oxide film entrapment, are preferably substantially free from these elements. In such castings any amounts of titanium and aluminum introduced from scrap or as deoxidants should be kept below 0.5%

Small additions of zirconium also benefit the tensile ductility and stress-rupture life of the alloys, and both zirconium and boron improve the ratio of notched to plain (unnotched or smooth-bar) tensile strength. It has also surprisingly been found that additions of zirconium improve both the weldability of the alloys and the stressrupture life of the weld metal deposited from filler material of matching composition. For these purposes the alloys can contain up to 1% zirconium and 0.1% boron, for example from 0.005% to 0.5% zirconium and 0.001% to 0.05 boron. Advantageously, the matching filler material has; clliigher zirconium content than the component being we e .4 Of the impurities and incidental elements such as residual deoxidants commonly found in nickel-chromium alloys foruse at high temperatures, silicon and manganese may be present in amounts up to 2% each, but the contents of these elements preferably do not exceed 1% each. The alloys are advantageously deoxided by means of an addition of magnesium, e.g., as a nickel-15% magnesium master alloy, leading typically to a residual magnesium content of 0.01% to 0.02% or 0.04%. Such residual magnesium contents improve the tensile ductility of the alloys. Nitrogen, which is introduced when the alloys are air-melted, may be present in amounts up to 0.15%.

Iron, which may be introdudced incidentally as a constituent of ferro-alloys used as a source of other alloying elements, may be present in amounts up to 12%, though it generally impairs the stress-rupture life, and for the best properties the iron content should not exceed 0.5 and is preferably as low as possible. However quite satisfactory results are obtained with amounts of iron up to 5%, e.g., from 2% to 4%, which enable the alloys to be produced at desirable low costs allowing niobium, tungsten and molybdenum to be introduced as ferro-alloys.

Two advantageous alloys according to the invention contain amounts of chromium, cobalt, molybdenum and tungsten in the following combinations of ranges:

(a) 24% to 27% chromium, 8% to 20% cobalt, and at least 5% tungsten, with percent Mo+ /z (percent W) from 4% to 7%; or,

(b) 28% to 32% chromium, 20% to 30% cobalt, and at least 4% tungsten, with percent Mo+ /z (percent W) from 2% to 4%.

The composition of each of these alloys also advantageously corresponds to points within the areas GHIJKLG and RSOTUR in FIGS. 1 and 2, respectively. They also contain 1.5 to 3.5% niobium, most advantageously 1.5% to 2.5% niobium, and 0.4% to 0.8% carbon. Within this range, the most beneficial carbon content depends on the manner of casting, as explained herein and should be lower than 0.65% if weldability under severe restraint is of importance. Beneficially the alloys also contain one or both of titanium and zirconium, e.g., 0.03% to 0.2% titanium and 0. 005 to 0.3% zirconium.

An especially advantageous composition is about 25% chromium, about 12% cobalt, about 9% tungsten, about 0.5 molybdenum, about 2% niobium, either about 0.1% titanium or about 0.01% zirconium and about 3% iron. The carbon content in normally 0.5 for centrifugally cast tubes and 0.65 for said or other refractory mould castings; the balance, apart from impurities, being nickel.

Numerous tests made on alloys of difierent compositions have shown that in order to obtain a good combination of satisfactory properties it is important that the contents of the constituents of the alloys are correlated according to the invention.

The alloys used in the tests were prepared by airmelting with a conventional addition of 0.3% manganese, 0.3% silicon and 0.03% calcium or magnesium (added as calcium-silicide or nickel-magnesium) as deoxidants, and cast as test-pieces in investment moulds.

Results of such tests are illustrated graphically in FIGS. 3 to 8 of the accompanying drawings and are set forth in the tables.

In FIGS. 3 to 8 the stress-rupture life in hours determined under a stress of 3 ton f./in. (2240-pound long tons per square inch) and at a temperature of 1000 C., is plotted on a logarithmic scale against the value of percent Mo+ /2(percent W). FIGS. 3 and 4 relate to alloys nominally containing 25 chromium and 0.75% carbon and show the effect of varying the cobalt content, at 2% niobium in FIG. 3 and 3% niobium in FIG. 4. FIGS. 5 and 6 similarly show the same effect when the chromium content is 30%. FIG. 3 also shows with a broken line the efiect of reducing to 1% the niobium content of alloys containing 10% cobalt and FIG. 4 similarly shows the effect of increasing the niobium content in these alloys to 4%.

The effect of varying the cobalt content is further illustrated by the curves in FIG. 7, in which stress-rupture lives at 3 ton l:'./in. and 1.8 ton f./in. at 1000 C. are plotted on a logarithmic scale against the cobalt content. The lives at 1.8 ton -f./in. at 1000 C. were obtained by extrapolation from results at higher stresses. The broken line relates to alloys nominally containing, apart from cobalt, 25% chromium, 2% niobium, 0.75% carbon and (Mo+ /2W) 5%, balance nickel, and the full lines to alloys nominally containing 30% chromium, 2% niobium, 0.75 carbon and 6% tungsten, balance nickel.

It will be seen that the best stress-rupture lives were provided by the alloys containing 25 chromium, 12.5% cobalt and 2% niobium at a content of percent Mo+ /2 (percent W) of from 4% to 7%, and by those containing It will be seen that Alloy A, which contained no niobium, gave a very poor result, while Alloys B and C with more than 5% niobiumwere again poor.

The elfect of varying the carbon content in alloys otherwise of the composition of Alloy No. 2 is shown by Table HI.

30% chromium, 25 cobalt and 2% niobium with TABLE III percent Mo+ /z(percent W) from 2% to 4%. Charpy V-notch impact Table I hereinafter shows the stress-rupture lives ob- Analyzed t i b liri me lit i tained other conditions as well as 3 ton f./in. at 1000 All carbon 3 at H A l 000 C 0y content on on s 1,000 .8 C. in alloys having these satisfactory combmations of Nov (percent) mum) imam) cast aimooled elements with varymg combmed molybdenum and tungsten contents. The Charpy V-notch impact strengths of the 3"": 8: 2g g2 g: 2:? 3:3 alloys at 20 C. are also given, some specimens being 10 0.62 260 660 2.9 2.2 tested in the as-cast condition and others after being held 3?? 33% 3g3 5 3 is for 1000 hour at 800 C. i on he 12 0.91 291 816 2.2 1.5 s s inmate prol ged atmg 13 1.14 145 440 2.2 1.5 in serum and then cooled m an.

TABLE I Stress-rupture Impact Percent Combined lite (hr.) strength 1 Mo+i w 0 Cr 00 W Mo Nb N1 (percent) (a) (b) (c) (d) (e) .75 25 10 6 1 2 Balance- 4 186 615 661 2.9 1.5 .75 25 10 6 2 2 ..d0.. 5 252 833 956 2.9 2.9 .75 25 10 9 0.5 2 -do.-- 5 397 1,080 1,184 2.9 1.5 .75 25 10 6 a 2 ...do-.-. 5 212 625 2.9 1.5 .75 25 10 6 6 2 -.do 9 111 396 454 1.5 1.5 .75 25 6 2 ..do 3 284 1,453

1 Impact strength or energy in foot-pounds (tt.-bl.). (a) 3.5 ton i./in. at 1,000 C. (b) 3.0 ton f./in. at 1,000 o. (c) 2.0 ton f./in.= at 1,050 C.

(d) As-cast. (e) After heating at 800 C. for 1,000 hr. and air-cooling.

Replacement of the 2% niobium in Alloy No. 3 by The remarkable improvement in stress-rupture strength 4% tantalum reduced the stress-rupture life at 3.5 ton f./in. at 1000 C. to 61 hours and the life at 3.0 ton f./in. at 1000 C. to only 137 hours.

In comparing the results in the tables with the curves in the accompanying drawings, it should be appreciated that the curves are based on the results of a large number of tests and that inevitably there is some scatter of actual test results above and below the curves. This arises both from experimental variation and from the fact that the alloys tested did not all have the same proportions of molybdenum and tungsten. Variation of the proportions of molybdenum and tungsten chiefly affects the optimum stress-rupture life, and this effect is shown by the set of curves in FIG. 8, which relate to alloys nominally containing also 25% chromium, 10% cobalt, 2% niobium, 0.75% carbon and balance nickel. It will be seen that the tungsten-free alloys have relatively poor properties and the stress-rupture lives increase as the proportion of tungsten increases. There is already a substantial improvement when the alloys contain at least 2% tungsten, while the alloys containing at least 5% tungsten are even stronger.

The eitect of varying the niobium content in alloys otherwise of the composition of Alloy No. 1 is shown by the stress-rupture and impact test results in Table II.

in Alloys Nos. 2 and 9 to 13 as compared with Alloy D containing less than 0.4% carbon shows the importance of controlling the carbon content according to the invention.

For the purpose of comparison, Table IV shows the composition of, and results obtained with, three commercial alloys that are used or proposed to be used for reformer tubes. The considerable improvement in stressrupture life obtained by means of the invention is clear from a comparison of Table I with Table IV.

1 Estimated from published data.

The results in Table V show the effects of additions of titanium, zirconium and/ or boron and of residual amounts of magnesium added as a deoxidant on the room-temperature tensile properties of alloys with two levels of carbon and iron contents. Apart from the elements shown in the Table V, the alloys nominally contained 25% chromium, cobalt, 6% tungsten, 2% molybdenum and 2% niobium, balance nickel. In this Table V N/P is the ratio of notched to plain tensile strength, E1 is the elongation on a gauge length of 5.65 times the square root of the area, and R of A is the reduction in area.

composition, apart from carbon, titanium, zirconium and aluminum in the amounts given in Table VII-A: chromium, 12% cobalt, 9% tungsten, 0.5% molybdenum 2% niobium, 3% iron, balance nickel. The stress-rupture and tensile properties of the alloys, of which Alloys to 34 were in accordance with the invention but Alloy K was not, are set forth in Table VII-A and VII-B.

TABLE V Composition (wt. percent analyzed) R A 0 Alloy number 0 Fe Ti Zr B Mg N/P El (percent) (percent) 1.24 4.5 7.2 1.20 13.5 11.0 1. a2 9.0 6.4 1. a0 6.8 5.0 1. 5.6 7.2 1.25 5.6 5.0 1.16 4 3.4 1.23 4.4 5.5 1 a3 2.6 5.0 1.22 6.0 6.6 1 17 6.0 7.0 1.21 4.9 9.6 1.13 2.3 1.1

Comparison of Alloy 14 with the five following alloys 25 TABLE shows respectively the increase in ductillty due to t1tamum (Alloy 15); the" improvement in both ductility and notched/ Percent Stress-rupture life (111.) plain tensile strength due to zirconium (Alloy 16); the Anoynumber C M (a) (b) (c) an improvement in notched/plam tens1le strength due to K boron (Alloy 17); the combin d efi t f zirconium a 30 a0 81% 13 3:3 &2 223 boron (Alloy 18); and the efiect of magneslum (Alloy 31 0. e4 301 1,186 1,617 331 boron (Alloy 18); and the efiectpf magnesmm (AHOY 35:11:33:32i"5ii.?.?i.:::::: iiii 31% 1,333 19). The undesirable eflect of mcreasing carbon on 34-- 0.42 0.44 notched/ plain tensile strength ratio and ductility is shown (99:35 ton at by comparison of Alloys 14, 20 and 26, and the eifect 35 (b)=2.7 ton i. 111. at 1,000 0. 2 (c)=2.5 ton mm. at 1,000" o. of magnesium is also shown by comparison of Alloy 0 ((3:24) tent/m, at 1,0506 and 21. The beneficial effect of zirconium on both TABLE VIIB Tensile properties-es east Tensile properties-1,000 hr./1,000 C. Impact strength, 0.1% 0.1% it.-lbs.

PS, U'Is, PS, UTS, ton ton El. Roi A ton ton El. RoiA As 1,000 hr./ L/in. (.lln. (percent) (percent) L/in. i./in. (percent) (percent) cast 1,000 0 19 38.0 13 1a 17. 41. 9 11 1 5. 4 5.1 18 37.9 12 10 18.6 39.9 as 2.0 6.1 3.0 19 88.0 6.9 6 20.9 41.7 2.2 2.0 4.7 2.9 20 37.5 7.9 10 20.7 40.5 2.7 2.0 7.6 2.9 18 39.0 15 12 19.5 41.9 5.2 5.0 8.0 4.3 19.3 39. 11 9- s PS=Prooi stress.

UTS Ultimate tensile strength.

notched/plain tensile strength ratio and ductility are again shown by the results for alloys 22 to 25.

The eifect of varying the iron content of the alloys is shown by the results in Table VI, which relate to alloys nominally containing, along with the amounts of iron shown, 0.75% carbon, 25% chromium, 10% cobalt, 6% tungsten, 2% molybdenum and 2% niobium, the balance being nickel.

TABLE VI Stress-rupture Impact strength life (hr.) (IL-lb.)

Alloy Fe number (percent) (A) (B) (C) (D) (A) =Stress rupture life at 3.0 ton Llin. at 1,000 degrees 0; (B;=Stress rupture life at 2.0 ton f./ln. at 1,050 degrees C. (O =Impact strength ft.-lb. as cast.

(D)=I1npact strength it.-lb. after 1,000 hrs. at 800 degrees C.

To demonstrate the properties of the alloys when cast in the form of centrifugally cast tubes, six tubes with dimensions of S-inches outside diameter and 1-inch wall thickness were cast in a commercial centrifugal casting machine.. Each of the melts for the tubes was deoxidized by an addition of 0.03 percent magnesium, as nickel-15 percent magnesium alloy, and had the following nominal The weidability of the alloys is surprisingly good, particularly when they contain zirconium or titanium or both and when their carbon content does not exceed 0.65%. The following example of the welding of alloys of the invention illustrates the advantage of the use of a filler material of high zirconium content on the stressrupture life and elongation of the weld metal.

The centrifugally-cast tubes of Alloys 30 and 32 were cut transversely, the cut ends prepared to a standard J- section and then welded together by argon-shielded arc welding using filler metal of matching composition, which accordingly was zirconium-free in the filler for Alloy 30 and contained 0.01% zirconium in the filler for Alloy 32. In each instance, sound welds were obtained with no evidence of heat-aifected zone cracking.

Stress-rupture tests on weld metal deposited from alloys of the same composition as Alloys 30 and 32. and froma third otherwise similar alloy containing 0.48% zirconuim produced the following results:

While the alloy of the invention is particularly useful for cast reformer tubes and other parts of hydrocarbon reforming plant apparatus, the alloy can also be utilized with advantage for other cast articles and parts of apparatus exposed to stress and corrosive attack at very high temperatures, for example, spinners for the production of glass fibers. Alloys in accordance with the invention can also be Worked by conventional hot-Working techniques such as rolling, forging and extrusion.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to Without departing from the spirit and scope of the invention, as those skilled in the art Will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. An alloy consisting essentially of 0.4% to 0.8% carbon, 24% to 27% chromium, cobalt in an amount from 8% to 20% and so correlated with the chromium content that the percentages of cobalt and chromium in the alloy are represented by a point within the area MNOPQM in FIG. 2 of the accompanying drawing, metal from the group consisting of molybdenum, tungsten and mixtures thereof in amounts such that the tungsten content is at least 5% and the sum percent M| /2 (percent W) is 4% to 7%, 1.5% to 2.5% niobium, up to iron, up to 1% of metal from the group consisting of titanium, aluminum and mixtures thereof, up to 1% zirconium, up to 0.1% boron and balance essentially nickel.

2. An alloy according to claim 1 which contains one or both of titanium and zirconium in amounts from 0.03% to 0.2% titanium and 0.005% to 0.3% zirconium.

3. A centrifugally-cast tube made from an alloy according to claim 1 in which the carbon content is 0.43% to 0.65%.

4. A welded structure made from an alloy according to claim 1 in which the carbon content does not exceed 0.65% and which contains titanium or zirconium or a mixture thereof.

5. An alloy according to claim 1 containing about 25% chromium, about 12% cobalt, about 9% tungsten, about 0.5% molybdenum, about 2% niobium, either about 0.1% titanium or about 0. 01% zirconium, and about 3% iron.

References Cited UNITED STATES PATENTS 3,212,886 10/1965 Freedman -171 2,513,469 7/1950 Franks et a1 75-171 X 2,537,477 1/1951 Mohling et a1 75-171 X 2,543,841 3/1951 Foley 75-171 X 3,069,258 12/ 1962 Haynes 75-171 UX 3,127,265 3/1964 Avery 75-171 3,316,074 4/1967 Laurent et al. 75-171 X 3,368,889 2/196 8 -Baumel 75-171 3,508,917 4/1970 Fleetwood et al 75-171 3,411,899 11/1968 Richards et a1. 75-17 1 3,466,171 9/1969 Fletcher et al 75-171 3,493,366 2/1970 Hopkins 75-171 X 3,552,952 1/1971 Shaw 75-171 FOREIGN PATENTS 654,354 6/ 1951 Great Britain.

637,436 5/1950 Great Britain.

612,001 11/ 1948 Great Britain.

773,871 5/ 1957 Great Britain.

617,194 2/ 1949 Great Britain.

674,023 6/ 1952 Great Britain.

681,247 10/ 1952 Great Britain.

HENRY W. TARRING II, !Primary Examiner US. Cl. X.R.

' g;;g=; UNITED STATES PATENT OFFICE 4 CERTIFICATE OF CORRECTION Patent N Dated August 1, 19.72

Invenwfls) Stuart Walter Ker Shaw and Peter John Penrice 7 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 47, for "MUVPQM" read -MUVPQM-.

Column 4, line 6, for "deoxided" read -deoxidi;ed

Column 7, line 32, delete "boron (Alloy l8) and the effect of magnesium (Alloy" Signed and sealed this 7th day of May 19714;

(SEAL) Attest:

. C. MARSHALL DANN Commissioner of Patents EDT-IARD II.FLETG1-IER,JR. Attesting Officer- 

